HP Serviceguard manual Single Cluster Lock, Dual Cluster Lock

Arbitration for Data Integrity in Serviceguard Clusters

Use of a Lock Disk as the Cluster Lock

Serviceguard periodically checks the health of the lock disk and writes messages to the syslog file when a lock disk fails the health check. This file should be monitored for early detection of lock disk problems.

You can choose between two lock disk options—a single or dual lock disk—based on the kind of high availability configuration you are building. A single lock disk is recommended where possible. With both single and dual locks, however, it is important that the cluster lock be available even if the power circuit to one node fails; thus, the choice of a lock configuration depends partly on the number of power circuits available. Regardless of your choice, all nodes in the cluster must have access to the cluster lock to maintain high availability.

Single Cluster Lock

It is recommended that you use a single lock disk. A single lock disk should be configured on a power circuit separate from that of any node in the cluster. For example, it is highly recommended to use three power circuits for a two-node cluster, with a single, separately powered disk for the cluster lock. For two-node clusters, this single lock disk may not share a power circuit with either node, and it must be an external disk. For three or four node clusters, the disk should not share a power circuit with 50% or more of the nodes.

Dual Cluster Lock

In an extended distance cluster, where the cluster contains nodes running in two separate data centers, a single lock disk would be a single point of failure should the data center it resides in suffer a catastrophic failure. In this case only, a dual cluster lock, with two separately powered disks, should be used to eliminate the lock disk as a single point of failure. The use of the dual cluster lock is further shown in “Use of Dual Lock Disks in Extended Distance Clusters” on page 26.

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